1. Field of the Invention
The present invention relates to a light emitting device capable of displaying gray scales using a time gray scale method, and a driving method thereof.
2. Description of the Related Art
As a known light emitting device, there is a light emitting device that is driven with an analog video signal, or a light emitting device that is driven with a digital video signal. In the case of using an analog video signal, gray scales can be displayed by controlling the luminance of a light emitting element with the analog video signal. Specifically, by controlling a gate-source voltage Vgs (gate voltage) of a TFT connected in series with the light emitting element using an analog video signal, a drain current value of the TFT that is supplied to the light emitting element, namely the luminance of the light emitting element is controlled.
However, in the case of the driving method using an analog video signal, the lower the level of the displayed gray scale is, the smaller the difference between the gate voltage Vgs and the threshold voltage Vth is required to be. In addition, a drain current of a TFT operating in the saturation region is proportional to the square of the difference between the gate voltage Vgs and the threshold voltage Vth. Accordingly, there has been a problem that a drain current is easily affected by variations of the threshold voltage Vth when adopting the driving method using an analog video signal.
Meanwhile, in the case of the driving method using a digital video signal, the gate voltage Vgs can be maintained constant, therefore, the difference between the gate voltage Vgs and the threshold voltage Vth can be set large. Accordingly, when displaying a low-level gray scale, a drain current is less easily affected by variations of the threshold voltage Vth than the case of using an analog video signal.
As one of the driving methods using a digital video signal, there is a time gray scale method by which gray scales are displayed by controlling emission periods of light emitting elements in pixels within one frame. Specifically, when displaying an image using the time gray scale method, one frame period is divided into a plurality of sub-frame periods. Pixels are controlled to emit light or no light in each sub-frame period in accordance with a video signal. With such a structure, the total length of the actual emission periods of light emitting elements in pixels within one frame can be controlled with a video signal, thereby gray scales can be displayed.
In the case of displaying an image using the time gray scale method, however, there is a problem that pseudo-contours are displayed in the pixel portion depending on the frame frequency. The pseudo-contours are unnatural contour lines that are often perceived when a middle-level gray scale is displayed using a time gray scale method, which are supposedly caused by a changing perception of luminance as a peculiar characteristic of the human vision.
The pseudo-contours include a moving-image pseudo-contour that is generated when displaying a moving image, and a still-image pseudo-contour that is generated when displaying a still image. The moving-image pseudo-contour is generated when a sub-frame period included in a certain frame period, and a sub-frame period included in the subsequent frame period are perceived as one continuous frame period by human eyes. That is, the moving-image pseudo-contour corresponds to an unnatural bright line or dark line that is displayed in the pixel portion when a gray scale, which has deviated from the desired gray scale to be displayed in a certain frame period, is perceived by human eyes. The generation mechanism of the still-image pseudo-contour is similar to that of the moving-image pseudo-contour. The still-image pseudo-contour is generated when displaying a still image, in which a visual point of humans slightly moves vertically or horizontally on the boundary between the regions where gray scales of different levels are displayed, which causes the still image to appear just as if a moving image is displayed in the pixels in the vicinity of the boundary. That is, the still-image pseudo-contour corresponds to an unnatural bright line or dark line that is displayed in a swaying manner in the pixels in the vicinity of the boundary between the regions where gray scales of different levels are displayed, which is caused by generation of the moving-image pseudo-contour.
In order to prevent generation of the aforementioned pseudo-contours, it is effective to increase the frame frequency. However, when the frame frequency is increased extremely higher, the length of each sub-frame period becomes shorter. Accordingly, the drive frequency of a driver circuit is required to be increased in accordance with the length of the shortest sub-frame period. Thus, it is not very preferable to increase the frame frequency when considering the reliability of the driver circuit.